Optimum frequency radio communication system



Sept. 12, 1950 J. K. DE ARMOND 2,521,696

OPTIMUM FREQUENCY RADIO COMMUNICATION SYSTEM Filed Jan. 24, 1949 4 Sheets-Sheet 1 Sept. 12, 1950 J. K. DE ARMOND 4 2,521,696

OPTIMUM FREQUENCY RADIO COMMUNICATION SYSTEM Filed Jan. 24, 1949 4 Sheets-Sheet 2 l l l I I l /gw' l i l i I.-- l l i i I l l 4 E Sept. l2, 1950 J. K. DE ARMOND OPIIMUN FREQUENCY RADIO COMMUNICATION SYSTEM 4 Sheets-Sheet 3 Filed Jan. 24, 1949 W www NNI 4 Sheets-Sheet 4 J. K. DE ARMOND OPTIMUM FREQUENCY RADIO COMMUNICATION SYSTEM Sept. l2, 1950 Filed Jan. 24, 1949 @miur Y. yThe invention described Patented Sept. 12, 1950 UNITED. STATES PATENT OFFICE OPTIMUM FREQUENCY RADIO COMIVIUNICATION SYSTEM James K. De Armond, Arlington, Va.

Application January 24, 1949, Serial No. 72,468 v i 7y claims. (C1. 25o-6) (Granted under the act of March s, 1883, as

l herein may 'be manufactured and used by or for the Government for governmental purposes without payment to me of any royalty thereon.

This invention relates -to radio communication systems and particularly to a system 1inwhich means are provided for automatically selecting an optimum transmission frequency.

Itis accordingly the-object of this invention to provide means in connection with a radio communication system for automatically selecting the -optimum frequency from a plurality of vpredetermined frequencies. y

At the present state of the art it isimpossible :to predict accurately what the optimum frequency will be at any given time for communication between'tworadio stations. Therefore determination of the optimumffrequency can only be done .by lperiodic sampling of the ionospheric reflections to the radio station with which it is desired to communicate. The exact frequency could be determined precisely only by means of panoramic transmission through the band Without hiatus; however this would cause undesirable interference vwith other services. By a judicious selection of spot frequencies for ytransmission v.the optimum k'frequency can be determined with'suificient precision to greatly improve the operating eliiciency of the communication system. In accordance 'with the invention this is accomplished by successive short transmissions on predetermined frequencies and by a comparison of these transmissionsA to determine which is the best of the frequencies tested. Communication between the two stations is then effected on the optimum frequency, preferably by means of high speed transmission from tape recorders or the like in order to complete the transmission before ionospheric changes can take place.

The invention employs the Afollowing basic steps in accomplishing the above: rv f. v y

(a)` The master station transmits on selected 1 spot frequencies in a band of frequencies in which the optimum frequency is known to exist, based on ionosphere propagation tables for the time of year and day and the period of the sun spot cycle. The signals are coded with the code identifying the particularly cooperating station with which the master station wishes to communicate. There will be an identifying code for each station l.with which the master station would be communicating from time to time.

, (b) The cooperatingstations are equipped with a standby receiver on each of the predetermined spot frequencies. The output of these receivers amended April 30V', 1928; 370 O. G5757) desired andthe frequencies on which the station is fed through a station code selector into an automatic signal strength analyzer in which a series of pulses is generated for each frequency, proportiona1 in number to the .strength of the signal received on that frequency. The series of pulses for each frequency is counted by a vstep counter for that frequency, When the signal strengths of all frequencies have been recorded on the counters the analyzer sweeps across the recorded signal strengths starting at the highest level and stepping down one step on each sweep until a contact is made indicating the frequency .of the strongest signal. The analyzer, having determined the optimum frequency, automatically .causes the transmitter at the cooperating station to transmit. a continuous signal onthat frequency `of sufficient duration to allow` ay semi-panoramic receiver at'the master station to act through a 'control circuit' to lock itself land the transmitter at the master station on theA optimum frequency. (c). The master station'then transmits to the ,cooperating station over the optimum frequency.

A (al)l As the, completion of transmission from the master station, the cooperating station automatically Avtransmits what traffic it may have for the'm'as'te'r station'.

'I'heldetails of va lpracticalembodiment of the invention will be explained in connection with the laccrmjipladny`ing drawings in which: p'

Figl l is a'block diagram of the controlling or ymaster station Vof the communication system.

Fig. 2 is a block diagram of a cooperationstationy of the communicating system.

Fig. 3 is a schematic circuit diagram of Fig. 1,

vand

Figs. 4a and 4b give ther schematic circuit diagrams of elements in Fig. 2. Thecontrolling or master station shown in Fig.

`f51 'comprises ajtransmitter l5 and a receiver I6 40:

simultaneously tunable to any one of a number 'ofldiiferent frequencies by means of frequency lselectors I1. ",In order vto initiate. transmission vbetween the master station and a cooperating station'it is necessary to know the identificationr code ofthe station with which communication is is standing by. The code is set into the coder 119 andthe various test frequencies, corresponding y tothe frequencies on which the cooperating station islstanding by, are set into a frequency selector in the'sequ'enc'e control I8.l The `sequence control 1.8v then automatically tunes the transmity ter through the frequency selector to. each of the test frequencies and, while the transmitter is on each frequencyvacts through coder lll to cause :quency by means of frequency selector 40.

the transmitter to radiate a signal consisting of a station identifying code followed by a short period of continuous signal of sufficient duration to permit evaluation thereof by the cooperating receivn ing station. Following transmission on all the test frequencies the sequence control continues to consecutively tune both transmitter and receiver through the frequency selector to the test frequencies with the coder I9 inoperative so that the transmitter is not keyed. During this period the cooperating .station analyzes the test frequencies and then radiates a continuous signal on the optimum frequency back to the master station. As receiver I6 sweeps over the various test frcquencies it will eventually be tuned to and receive the return transmission on the optimum frequency from the cooperating station.y Upon reception of this signal the sequence control acts to lock transmitter and receiver' on this frequency and to ,connect transmitting recorder 2B to transmitter I for high speed transmission of messages or other intelligence from the master station to the cooperating station. At the end of the transmission a special signal starts the receiving re- .corder to receive traic from the cooperating station. At the conclusion of recording traic from the cooperating station the sequence control automatically returns to its original or starting condition.

The block diagram of a station to cooperate 'with the above described master station is shown in Fig. 2. The station shown comprises four channels, land therefore can stand by on four test frequencies. A greater number of test frequencies may be accommodated by increasing the number of channels. Each Vof receivers 22 through 25 stands by on one of theA four test v'freformer produces a series of equally spacedA pulses i proportional in number to the received amplitude of the short test transmission that follows the identifying code on each frequency, and applies these pulses to signal strength analyzer 34 where the number of pulses is recorded. After the number of pulses so produced has been recorded for each of frequencies f3, f4, f5 and fe, the analyzer determines for which channel the highest number of pulses was recorded and acts through the recorder and transmitter control for "that channel to set transmitter 4| to the optimum fre- The analyzer further acts through the lreceiver and transmitter control of the selected channel to initiate a short period of constant signal trans- 'and transmitter control acts to disconnect fthe receiving recorder'from the receiver and to conneet the' transmitting recorder 42 to the trans-Y mitter for the transmission of outgoing traffic i Figs. 3, 4a and 4b show the details of suitable embodiments of these elements. Referring to Fig. 3, which shows the details of the elements shown in block form in Fig. 1, the sequence control I8 comprises a stepping switch Si having two banks of contacts A and B and a second stepping switch S2 having three banks of contacts A, B and C. The revolving contacts in switch S1 are moved one step in unison for each pulse of voltage applied to stepping coil 50. f Similarly the revolving contacts of switch S2 are moved one step in unison for each pulse of voltage applied to stepping coil 5I. Trip coils 52 and 53, each designated T, are provided for returning'the switches Si and S2 to their starting positions. Each contact of bank B of switch S1 has connected in series therewith a push-button switch of the type that remains open after being depressed. Each of these switches represents a frequency to which the transmitter and receiver may be tuned. Bank A of this switch is used to change the transmitter and receiver frequency as will be explained later. A pulser 54 is provided, which produces voltage pulses at a constant rate, for example, one per second, to operatey switches Si and S2. Thesequ'ency control further contains a number of relays which cooperate with switches Si and S2 in a manner which will be explained later in discussing the operation of the master station.

Coder I9 consists of a rotating switch having aroundits periphery two series of equally spaced contacts 55 and 56, a continuous strip contact 51 and an additional contact 58. The revolving lcontact of this switch is driven by a' motor 59 at a speed of 12 revolutionsr per minute, so that the contact makes one revolution in ve seconds. The revolving contact therefore requires about one second to pass over the series of contacts 55, one second to pass over the series 56, about two seconds to pass over the contact 51 and the re maining one second in passing over contact 58 andthe blank spaces. A conducting ring 60 is also provided which contacts the revolving contact 6I in yall positions except a short distance on either side of its starting position. Each of the contacts in the series 55 and 56 has connected in series therewith `a push-button switch of the type that when pushed-remains closed.

,The frequency selectorv I 'I'is associated with transmitter I5 and receiver I6 and operatesupon -the application of a positive lvoltage to the proper circuit thereof to set both transmitter and receiver to one of a number -of frequencies, in this case ten. These circuits are indicated by the ten connections numbered fi through fio: The specific details of the frequency selector are unimportant `andare not shown. lien double-pole "relays, all identical to relay 62,-are provided for applying positive voltage through their upper contacts lto one'of vthe circuits f1 through )'10, the

Arelays for fi and fio'only being shown'. vThe actu- 'ating coils lof these ten relays are connected in 5. order' to terminals i through l0 in bank A of switch S1 so that the position of this switch determines the frequency to which the transmitter and receiver is set. The lower contacts of relay 62 and the other nine similar relays are connected in parallel and to the actuating coil of relay 63 for applying a positivevoltage thereto upon actuation of any of these relays for reasons which will be apparent later. rlhe transmitter l5 is keyed by the application of a positive potential to keying i circuit 65, A relay 64 is provided for blanking the receiver I8 upon actuation of the relay. This may be simply accomplished by short-circuiting a suitable point in the receiver or by applying a high negative bias to one of the tubes in the receiver. The actuating coil of relay 64 is connected to the keying circuit 55 so that the application of a keying voltage to the transmitter renders the receiver inoperative. Relays Gt and 6l are associated with transmitting recorder and receiving recorder 2I,vrespectively, and render these devices operative while the relays are in an energized condition.

In order to communicate with another radio station throughthe master station shown in Fig. 3 it is necessary to set the coder to produce the identifyingf code of the station withwhich communication is desired. In the embodiment shown a two-digit pulse code is employed and the coder is shown as having been set to the code 35 by depressing the first three buttons in contact series 55 and the first nve buttons in contact series 56. The frequencies on which the cooperating station is standing by, must also be known and these frequencies must be set into the system by depressing the proper buttons in connection with the contacts Aof bank B in switch S1. In the embodiment shown, the switch S1 has been set for test transmission on the frequencies f3, fi.' f5 and f6. To initiate operation oi the master station the starting switch t8 is momentarily depressed to apply a positive voltage to stepping coil 5B of switch S1, thus stepping this switch to the contact l position. With the rotating contact of bank B on contact i, a circuit is made from the pulser 236i through the Lipper contact of now released switch et, contact SzAi (this symbol indicating switch Sz, bank A contact I) and contact SiBl to the actuating coil 56, so that the next pulse from thepulscr steps switch S1 to the contact 2 position. ri`his process continues until the first contact in bank B of switch S1 having an open circuit in series therewith is reached, which, in this case,-is contact 3. When Contact 3 is reached the circuit to coil 5S is broken and switch S1 rests in this position. The arm of bank A switch S1, now resting on contact 3, applies a positive voltage to the actuating coil of the relay associated with the f3 circuit of frequency selector il. Actuation lof this relay causes the transmitter and receiver to be set to frequency f3 and also provides a voltage to the actuating coil of relay B3 which actuates this relay and applies a positive voltage from. contact S20! through the upper contact of relay 63 to motor 5t, starting rotation of contact 6l vin the coder.v Soon after this contact begins to revolve, it makes contact with ring 60 which likewise applies a positive voltage to the motor 59 for l"reasons which will be apparent later. Contact tl, in passing over the first three contacts in series 55, keys the transmitter with three pulses and, in passing lover the rst ve contacts of series 56, keys the transmitter with ve pulses, thus transmitting the identifying code of the station-'with which communication is desired. As

Gf' the contact 'passes over strip 51 the transmitter l5 is-continuouslyfkeyed for about two seconds during which time" the cooperating station receives and measures the strength of this signal. When contact 5I reaches contact 58 a positive potential is applied to stepping coil 5U of switch S1 and steps this switch one step. The ring 60 insures that motor 59 continues to run until contact 6i has reached its starting vposition should the relay B3 open due to the connection at contact S1A3 being broken. The switch S1 is now stepped by pulses from the pulser y54 in the manner previously explained until the next contact having an open circuit in series therewith is reached, which in the embodiment shown is con- 4 tact f4. When this contact is reached the transmitter and receiver frequencies are set to .f4 and the coder again rotates through a complete revolution causing the transmitter to send out the identifying code signal followed by a test transmission as in the case of frequency f3. In a similar manner test transmissions are madev on frequencies f5 and fs.

At the conclusion of transmission on frequency f6 the revolving contact SI of coder |49 makes connection with contact 58 stepping the switch S1 to contact' l. Impulses from puiser 54 then actuate stepping relay 5d of switch S1 until contact Hvv is reached. There is now a circuit from' the lpulser through the upper contact of starting switch 68, contact SzAi and contact SiBll to the stepping coil 5i of switch S2 causing this switch lto rotate one step. A connection now exists from the pulser through the upper contact of starting switch t8, Contact S2A2 and upper ycontact of relay 69 to stepping coil 5Fl so that switch S1 begins to rotate at the rate of one step per second. There is also a connection from the output of receiver I6 through contact S2132 to the actuating coil of relay 68. So long as relay 69 remains unenerg'ized the switch S1 continues to rotate and acts through bank A to consecutively tune the transmitter and receiver to the frequencies of f1 through fio for one second intervals each in recurring cycles. While the receiver is being swept over the various frequencies as described above, the station with which communication is desired is analyzing the test transmissions on the four selected frequencies and will eventually transmita signal back to the master station on the frequency found to produce the strongest signals in a manner which will be described later. When this return transmission occurs the receiver It, in switching over all the frequencies, will eventually become tuned to the return transmission and relay 59 will be energized. This relay is of the delayed actuating type, as indicated by the legend DA, with the delay in this case being suiicient to prevent operation of the relay in response to short transients, such as noise, but to allow operation in the presence of a sustained signal orduration amounting to the substantial part of a second. Actuation of relay $9 causes a 1 positive potential to be applied through the lower contact of this relay to stepping coil El rotating switch S2 one step to the contact`3 position. Actuation of relay 89 also blocks the circuit through its upper contact to 'stepping coil 50 thus causing switch Si to remain in its vlast position, and the frequency of transmitter l5 and receiver I6 to be locked on the received frequency through the appropriate contact in bank A of switch S1. It will be assumed for purposes of illustration that the return transmission was on f3 and that switch S1, as a result, remains in its contact 3 position thus locking transmitter l5 and receiver I6 onthis frequency.

switch S2 through three more positions.

i The-.output of receiver l is now connected .through terminal sans to the actuating con of normally ,closed -relay 10 thus opening the contacts of this relay and preventing the application o f a positive potential through contact S2C3 and the lower contact of relay 3 to relay GS of transmitting recorder 20. The operation of relay 63 should be delayed sufficiently to insure prior actuation of relay 1 0. So long as the return test transmission from the station with which communication is desired continues, and this period should be of suiiicient length to allow at least two rotations o f switch S1, the relay 1B remains actuated and transmitting recorder 2s] remains deenergized When the return transmission ceases however, the relay 1 0 is de-energized and a positive potential is applied to relay 66 to initiate operation of the recorder. The release .of relay 1D should be slightly delayed, as indicated by the legend DR, so as to prevent its release by transient iluctuations of short duration in the receiver signal strength. The outgoing traflic from the transmitting recorder is now applied through isolating circuit 1| to the keying circuit E5 of the transmitter.

The recorder will ordinarily be a tape recorder or a similar device for high speed transmission and, in .the system shown, it is arranged for a code of three long pulses to follow the end of the recorded matter.

delayed to .prevent response tc the recorded trafiic but which operates rapidly enough to respond .to the three terminating pulses. The isolating circuit 1I prevents the application of voltages Actuation of relay 12 from coder 9 to relay 12. by the three terminating pulses from the transmitting recorder causes three positive pulses to be applied to stepping coil 5l of switch S2 and `this switch to step three more positions. There now exists a connection from the output of receiver IG through contact S2B6 to the actuating coil of relay 13 and to the input circuit of receiving recorder 2l. Also positive potential is applied through Contact SzCS to relay 5l' thus initiating operation of the receiving recorder which then records vthe return .traffic from the station with which communication is being had. This transmission is likewise terminated by a code of three long pulses which actuate delayed relay 13 to step Positive potential is then applied through contact S209 to the actuating c oil of relay 14 which, upon actuation, applies positive potential to trip coils 5 2 and 5 3 allowing .switches Si and S2 to return to their original starting positions,

A block diagram of a radio station for coop- .erating with vthe above described master station has been shown in Fig. 2. Fig. 4a. shows the cirvcuitdetails for one channel .of the block diagram in Fig. 2. Since all channels are alike except for the frequencies to which they are tuned a description of one ot' the -channels will be sutlicient.

In Fig. ea the channel shown is assumed to be the one tuned to the frequency f3. The test transmission .on frequency js is received by receiver 22 and applied through the upper contact of -relay and upper contact of relay 15 to stepping coil 11 of stepping switch S3 in the station .three pulses of this code upon application to coil 1l rotates switch S3 through three steps. A positive potential is then applied through .contact rEhe output of the transmitting recorder is applied to relay 12 which is suiciently anali/zer ac comp `SsBB to the actuating coil of relay 1E. A circuit now exists from the receiver through upper contact of relay 15, lower contact of relay 16, contact S3A3 and the upper Contact of relay 18 to stepping coil fig of stepping switch S4. The next five pulses of the code therefore operates the step switch S4 through ve positions. Positive potential is then applied through contact S435 to the actuating coil of relay 18 thus closing this relay and providing a direct circuit from the receiver to the actuating coil of relay 8U through the upper contact associated with the lower armature of this relay. This circuit cannot be completed for any code .other than 35 for, with the rotating contacts of bank B of switches S3 and S4 resting on any contacts other than 3 and 5 respectively, positive potential is applied to trip coils 8l and 82 thus returning these switches to their starting positions. The action of trip coils 8| and .82 should be delayed suiiiciently to prevent action .thereof unless a voltage is applied thereto for a period exceeding the normal period between code pulses.

The function of the pulse former 3i! is to generate a series of equally spaced pulses proportional in nuns-ber to the strength of the signal applied thereto. For this purpose a mirror galvanometer 8 3, a light soiufce 8,4 and a bank of photoelectric cells connected in parallel to the input oi the ampliiier 86, are provided. These elements are so arranged that a spot of light from the light source is reflected by the mirror and falls on the photoelectric cells 85. The light source is provided by a normally opened shutter which may be closed by the application of a signal to coil 81. As already explained, the code portion of the test transmission from the master station is followed by continuous test signal of about two seconds duration. This signal actuates relay 8B which is of the delay release type and is adjusted to hold foi slightly longer than the ,duration of the signal, or slightly more than two seconds. The signal is then applied through the lower armature and contact of this relay to the mirror galvanorneter 8 3 and the shutter coil 81. Actuation of relay 8 6 also causes a positive potential to be applied through the upper armature and contact thereof to the actuating coil of relay 83. Actuation of this relay applies a positive potential through its upper contacts to the light source 84, for energizing this source. The mirror of galvanorneter 83 is rotated clockwise through an angle proportional to the strength of the signal applied thereto. During the presence of the signal no light falls on the mirror since the shutter of the light source 842 is held closed through the action of the signal on coil 81. At the end of the test signal coil 8? is de-energized and light Afrom the light source 2f" falls on the mirror of the ealvanonieter which begins to return to its zero position. In returning, a light spot is swept over the photoelectric cells producing one pulse for each cell passed over. The actuation ojf relay 3 8 should be slightly delayed to prevent energization of light source 84 before closure of its shutter. Also .the release of this relay should be sufnciently delayed to allow time for the galvanonieter to return to its zero position.

The signal strength analyzer records the number o pulses produced in each channel by the test transmission and selects the channel having the maximum number of pulses indicating the strongest signal. The manner in which the this will be explained later ls assumed that the transbut, .for the pren mission on frequency fs produces the maximum number of pulses and that this frequency has been selected `by the analyzer as the optimum frequency. The analyzer then operates to yapply a positive potential to the terminalv h of the transmitter and recorder-control fili in Fig. 4a thus energizing the actuating coil of relay ils and actuating this relay. Positive potential is now applied through the upper contacts of relay t9 to the actuating coil of relay 9d. Relay 9G is designed to close upon energiaation and then to re-open after a fixed period. rlhe period in this case should be sufficient to allow two rotations of switch S1 in Fig. 3, or about'25 seconds minimum. Actuation of relay Sill causes a positive potential to be applied to keying connection 3| of transmitter 4| thus keying the transmitter for 25 seconds or more and producing the previously described return transmission on the .optimum frequency to permit the master station to adjust itself to this frequency. A positive potential is also applied through upper contacts of relay 8d to the actuating coil of relay l thus connecting the output of receiver 22 through the lower con-- tacts of this relay to the actuating coil of relay 92 and through the lower contacts of rela-y all to the receiving recorderv Sil thus placing the recorder in a position to receive the traffic from the master station when the test transmission `from transmitter lil is completed. A connection 9S is provided between the keying connection -ll of transmitter 4| and the actuating coil og relay 94 to blank receiver 22 when transmitter lll is keyed.

As previously explained, the transmission from the master station ends with a code of three long dashes. The relay 92 which is delayed sufliciently to prevent actuation by the normal traffic responds to these three long pulses to step switch S18 through its stepping coil 94 to the contact 3 position. Positive potential is then applied through contact S183 to relay 95 to close this relay and energize transmitting recorder 42. The transmitting recorder is connected through the middle contacts of relay 89 and the isolating circuit 96 to the keying connection 9| of the transmitter which has previously been set to the optimum frequency f3 by the application of a positive potential to the f3 terminal of frequency selector 40 through the upper contacts of relay 89. The transmitter now sends the recorded traflic at the cooperating station to the master station. The signal from the recorder is also applied through the middle contacts of relay 89 to the actuating coil of relay 91. The outgoing traic, like the received traiic, terminates in a code of three long pulses to which delayed relay 91 responds to step switch S18 through three more positions to contact 6. Positive potential is now applied through S186 to trip coil 98, to trip coils 8| andv82 and, through terminal g, to all the trip coils in the signal strength analyzer shown in Fig. 4b. The action of trip coil 98 should be delayed suiiiciently to insure actuation of all other trip kcoils before it returns switch I8 to its starting position.' This action completes one cycle of operation of the cooperating station and the station is now in condition to handle thenext transmission initiated by a master station.

The details of the signal strength analyzer 34 of Fig. 2 are shown in Fig. 4b. The analyzer comprises a two bank stepping switch for each of the channels in the cooperating station. The analyzer shown is for use with the four channel station shown in Fig. 2 and comprises four'steping switches S5, Se, S7 and Sa, one for each of frequen'cies f3, f4, f5, and fs. It will beassumed that switch "S5 in this case is associated with the f3 channel shown in Fig. 4a. The output fromthe amplier 86 in pulse former 30 of Fig.` 4u isl applied through terminal e to the stepping coil '99 of switchk S5 and rotates this switch throughfa number' of steps equal tothe numberof pulses. The terminals l, m, '11., 0,40, and q of switches Se, S7 and Ss are connected to thevpulse formers', transmitter and recorder controlsl 'of the remaining three channels in the same manner that the terminals eand h are connected to thehchan'nel shown in Fig. 4a, and each of switches Se, S7and Ss is stepped through a number ofpositions de',- pending on the number of 'pulses received from itslassociated pulseformer. On'thel'lrst position of any of y these switches a connection isf'nilade from ground through vcontactsAl andlBl'toq'actuating coil of relay 90. l.Thus theiirst switch to be rotatedvone step causes actuationoftliisrelay. Relay |90`starts a timing'device ||J|',lhaving contacts which close after a predetermined-time interval, the length of which must be greater ythan the total time` required for switches S5 through Ss to recordthepulses from the variouschan'nels. At the end of this interval.` the lcontactsfof.` timer ||l| are closedand 'initiate' action of pulse;` 4.0.2.

The signal strength analyzer `also comprises nine additional two bank, stepping `switches-S9 through S17, 'each of which corresponds toaLsig- .nal strength from the strongestrecordable"signal (710 pulses) presented .by `switch S9 jtol the weak- 'est recordable signal (2 pulses) representeclby switch S17. Accordingly contactsZ, 3,.,4 and-1.5 of bank A of switch S9 areconnected to the, Bill contacts of switches Ss through .Sarespectively Similarly contacts 2, 3, 4 and 5 of switch rSicfare connected to the B9 contacts of switche'ssQSs through Ss, respectively, andA similarly downlthe line to switch S17 which-has 'contacts 2|. 3 ,l 4. and/i5 of bank A connected 'to 4 theB2i"contactsfor switches S5 through Ss respectively; (Terminals through 5 of the B bankjo'f 1switches S9 through S17 are connected together yandto vtheir associated stepping coil. The B6 contact in each ofswitohes S9 through S16 is connected tothe rotating oo nltact in bank 2Bl of the next succeeding switch;

The pulser |02 producesequally spaced pulses the frequencyV of which maybe asjhigh asv the switch S9 through S17 are able to follow.' Upon energizationof the lpulser by the closingv`v` of vthe contact 'in timer |0`| pulsesare applied through normally closed contacts of relay |03 and through contactSsBl to the stepping coil of switch S9. As long as the relay |03 remains closed switch 'Ss continues to step until contactr is reached due to the fact that contacts through 5 are connected" together. When contact 6 has been reached pulses are thenv applied to the revolving contact in bank B of switch S10 and uponthis switch reaching contact. 6' switch S11 begins to rotate, and so on until the revolving contact in bank A of `one of switches Ss through S17 reaches a stationary contact'connected' to a contact in the B bank of one of switches S5 through Ss. with vwhich contact has been made; yFor example,

assume that seven pulses are received from the pulse former in channel f3 so that switch S5 rests on contact 1, and that no more than sevenpulses are received from the other channels so` that switches Se, S7 and Sa do not rest on any contacts above the seventh. Then when the revolving contact in bank A of switch S12 reachescontact 2 in this bank a circuit is completed from' ay source of positive potential'through the actuatingcoil 11 of relay ID3, contact S12A2, contact S5B'I, contact SsAl, terminal h (Fig. 4a) and the actuating coil of relay 8 9 to ground. The resulting energization of relay |03 opens its contacts and prevents further rotation of switch S12 from the contact S12A2 position. selects-the f3 channel with which switch S5 is associated and initiates operation of this channel to receive and transmit with the master station on the optimum frequency. switches restedon contacts beyond the seventh then the channel corresponding to the switch which rested on the highest contact would have been selected. For example, had switch S7 rested on the'No. 9 contact then when the revolving contact in bank A of switch S10 reached stationary `contact 2 a circuitwould have been completed in the, manner described `alcove forchannel fs, through the actuating coil of relay |03 and the terminal Ao thus applying a positive potential to the actuating coil of the relay corresponding to ,relay l8,9 in the channel associated with switch S7 and this channel would have been selected. It is therefore seen that the Av banks of switches S9 through Sm correspond to signal strengths and that contacts 2, 3, 4 and 5 in each of these correspond to the four channels of the cooperating radiostation, and that by consecutive operation signal strength analyzer, thus returning the stepvpingv switches therein to their starting ,positions as already mentioned.

I claim as my invention:

1. A radio communication system comprising a Amaster station and a, cooperating station, means in said master station capable o f transmitting and receiving on a plurality of different carrier frequencies, means in said master station cooperating with said transmitting means for effect- ,ing consecutive short Atest transmissions on each o f said plurality of carrier frequencies, means in said cooperating station capable'of transmitting and-receiving on ,said` plurality of different carrier frequencies, means in Ysaid cooperating station for analyzing said test transmissions as to signal strength to determine the optimum frequency of Vsaild plurality of carrier frequencies for transmission between said stations and for adjusting the transmitting and receiving means of said .cooperating station to lsaid optimum frequency,

meansv in said cooperating station for. acting on the transmittingmeans thereof to effecta short return transmission to said master station on said optimum frequency, and means in said Vmaster station responsive to said return transmission for ,adjusting the transmitting and receiving means of .the master station to the frequency of said return transmission. f

2. A radio .communication system for. automatically effecting transmission between two stations onthe optimum of a Vplurality of frequencies, said system` comprising a master station and a cooperating station, said master` station containing a transmitter'and .a receiver, a fre- Further, the actuation of relay 89 Had the other quency selector in said master station for adjusting said transmitter and receiver to any one of a plurality of frequencies, sequence control means in said master station cooperating with said transmitter and frequency selector for effecting consecutive short test transmissions on each of said plurality of frequencies and, at the conc1usion of said test transmissions, for adjusting said receiver consecutively to each frequency on which a test transmission was made in re-peating cycles, a receiving channel in said cooperating station for each of said test frequencies, transmitting means in said cooperating station capable of :being adjusted to any of said plurality of frequencies, analyzing means in said cooperating station for analyzing the test signals from the various receiving channels as to signal strength to determine the optimum frequency of said plurality of frequencies for transmission between said stations, for selecting the receiving channel corresponding to said optimum frequency and for adjusting said transmitting means to said optimum frequency, means in said cooperating sta.- tion for keying said transmitting means to produce a short return transmission to said master station on said optimum frequency, and means in said master station effective upon reception of said return transmission to lock the receiver and transmitter in said master station to the frequency of said return transmission.

3. Apparatus as claimed in claim 2 in which said sequence control means comprises a stepping switch having two similaibanks of contacts, each bank comprising a plurality of stationary contacts and a movable contact, electromagnetic means for moving said movable contacts in unison one step for each electrical pulse applied thereto, a source of electrical pulses, means for applying the pulses from said source to the movable contact of one .bank of contacts, mea-ns for connecting each of the stationary contacts of said one bank through ya circuit interrupting means to said electromagnetic means, and means connecting the movable and stationary contacts of said other bank in circuit with said frequency selector for actuating said frequency selector.

'4. Apparatus as claimed in claim 3 in which means including the last stationary contact in said one bank of contacts are provided for operating when connection is made with said last contact during the first cycle of operation of said stepping switch to connect the electrical pulses from said source through a signal actuated switching means to said electromagnetic means to cause repeated cycling of said sequence switch, and for connecting the actuating coil of said signal actuated switching means to the output of said receiver whereby reception of said return transmission interrupts the circuit between said pulses and said electromagnetic meansI and stops said stepping switch, thus holding said ,transmitter and receiver on the frequency of thc received signal.

5. Apparatus as claimed in claim 2 in which said analyzing means comprises a pulse generating means connected to each of said receiving channels for generating a series of equally spaced electrical pulses proportional in number to the test signal strength in the associated channel, pulse counting means for recording the number of pulses produced Iby each `pulse generating means, and means cooperating with said pulse counting means for adjusting the transmitter in said cooperating station to the frequency of the reflected by said mirror on to said photoelectric cells, means connecting said photoelectric cells in parallel and to an output circuit, means for applying a signal to said galvanometer to deflect same, and means operative at the termination of said signal for unblanking said light source whereby the resulting beam of light passing over said photoelectric cells `as the galvanometer returns to its zero position results in a series of pulses in said output circuit proportional in number to the strength of said signal.

7. Apparatus as claimed in claim 6 in which l' said pulse counting means comprises a pulse counting stepping switch for each of said receiving channels, each of said pulse counting stepping switches having contacts equal in number i to the highest possible number of pulses in said series of pulses, and meansl for connecting the output of the pulse generating means for each channel to the actuating circuit of the corresponding :pulse counting stepping switch, and in which said means cooperating lwith said pulse counting means for adjusting the transmitter in said cooperating station to the frequency of the channel for which the maximum number of pulses was recorded comprises a plurality of additional stepping switches equal in number to one less than the number of contacts on each of said pulse counting stepping switches, each of said additional stepping switches having as many contacts as there are pulse counting stepping switches, .means connecting each contact in each additional stepping switch to a contact other than the first in one of said pulse counting stepping switches with they contacts in any one additional 14 switch being connected to corresponding contacts in said pulse counting stepping switches, an additional pulse producing means, means operative upon the application of the rst pulse to any of said puls-e counting stepping switches to initiate operation of said additional pulse producing means a fixed time interval after the application of said rst pulse, means for applying the pulses produced by said additional pulse producing means to the actuating circuits of said additional stepping switches inor'der beginning with the one associated with the highest numbered contacts of said pulse counting stepping switches and ending with the one associated with the second contacts of said pulse counting stepping switches, means associated :with said additional stepping switches for opening the circuit from said additional pulse producing means upon the completion of a circuit through one of said additional stepping switches and one of said .pulse counting stepping switches, and means operative upon completion of the above mentioned circuit for adjusting the frequency of the transmitter in said cooperating station to the frequency of the channel with which the pulse counting stepping switch through which the circuit Iwas completed is associated.

JAMES K. DE ARMOND.

REFERENCES CITED The following references are of record in the nie of this patent:

UNITED STATES PATENISv Number Name Date 1,794,393 Brown et al. Mar. 3, 1931 1,913,495 Matte June 13, 1933 2,125,977 Zworykin Aug. 9, 1938 2,260,160 Benning Oct. 21, 1941 2,280,822 Hansell Apr. 28, 1942 2,326,290 Dickieson Aug. 10, 1943 2,363,583 Gilman Nov. 28, 1944 2,425,614 Goddard Aug. 12, 1947 

